1. Field of the Invention
The present invention relates to the field of integrated circuits. More particularly, this invention relates to monitoring an operating parameter within an integrated circuit.
2. Description of the Prior Art
It is known to provide an integrated circuit with one or more monitoring circuits which seek to provide monitoring information regarding operating parameters of the integrated circuit. A typical operating parameter to be monitored is operating temperature. Other operating parameters which may be monitored include an operating voltage. This information can be used to ensure correct operation of the integrated circuit and may, in some circumstances, be used to adjust the operation using a feedback mechanism.
It is known to provide ring oscillator circuits in which the oscillation frequency gives an indication of the operating temperature of an integrated circuit. As the integrated circuit heats up, the transistors making up the inverter chain within the ring oscillator will operate more rapidly and accordingly the oscillation frequency will increase. A problem with such mechanisms is that the relationship between oscillation frequency and temperature can be complex and the ring oscillator may require relatively complicated biasing circuits and/or analogue outputs. Also, to obtain a low enough frequency to be measurable a ring oscillator needs to comprise a relatively large number of gates.
As process geometries diminish in size, local variation of basic MOSFET characteristics become so large that simple chip-level guard-banded designs become heavily over designed and inefficient. In addition, key systematic effects, such as implant shadowing or stress engineering effects multiply the number of permutations of MOSFET implementations that should be monitored. Owing to such effects, MOSFET characteristics have become location and context-specific. Accordingly, relying on a simple set of boundary-provided wafer acceptance test MOSFET parameters will be less accurate than system-on-chip embedded MOSFET monitors due to context-dependent effects and local variations that cannot be monitored from the distant areas where wafer monitors are provided. Accordingly, there is a need for a small and low-power monitoring circuit for embedded MOSFET monitoring as well as silicon acceptance testing, performance binning and adaptive circuits.